% File src/library/parallel/man/detectCores.Rd
% Part of the R package, https://www.R-project.org
% Copyright 2011-2018 R Core Team
% Distributed under GPL 2 or later

\name{detectCores}
\alias{detectCores}
\title{Detect the Number of CPU Cores}
\description{
  Attempt to detect the number of CPU cores on the current host.
}
\usage{
detectCores(all.tests = FALSE, logical = TRUE)
}
\arguments{
  \item{all.tests}{Logical: if true apply all known tests.}
  \item{logical}{Logical: if possible, use the number of physical CPUs/cores
    (if \code{FALSE}) or logical CPUs (if \code{TRUE}).  Currently this
    is honoured only on macOS, Solaris and Windows.}
}
\details{
  This attempts to detect the number of available CPU cores.

  It has methods to do so for Linux, macOS, FreeBSD, OpenBSD, Solaris,
  Irix and Windows.  \code{detectCores(TRUE)} could be tried on other
  Unix-alike systems.
}
\value{
  An integer, \code{NA} if the answer is unknown.

  Exactly what this represents is OS-dependent: where possible by
  default it counts logical (e.g., hyperthreaded) CPUs and not physical
  cores or packages.

  Under macOS there is a further distinction between \sQuote{available in
    the current power management mode} and \sQuote{could be available
    this boot}, and this function returns the first.

  On Windows:
  Only versions of Windows since XP SP3 are supported.  Microsoft
  documents that with \code{logical = FALSE} it will report the number of
  cores on Vista or later, but the number of physical CPU packages on XP
  or Server 2003: however it reported correctly on the XP systems we
  tested.
  % https://msdn.microsoft.com/en-us/library/ms683194%28v=VS.85%29.aspx

  On Sparc Solaris \code{logical = FALSE} returns the number of physical
  cores and \code{logical = TRUE} returns the number of available
  hardware threads. (Some Sparc CPUs have multiple cores per CPU, others
  have multiple threads per core and some have both.)  For example, the
  UltraSparc T2 CPU in the former CRAN check server was a single
  physical CPU with 8 cores, and each core supports 8 hardware threads.
  So \code{detectCores(logical = FALSE)} returns 8, and
  \code{detectCores(logical = TRUE)} returns 64.

  Where virtual machines are in use, one would hope that the result
  for \code{logical = TRUE} represents the number of CPUs available (or
  potentially available) to that particular VM.
}
\author{
  Simon Urbanek and Brian Ripley
}
\note{
  This is not suitable for use directly for the \code{mc.cores} argument
  of \code{mclapply} nor specifying the number of cores in
  \code{makeCluster}.  First because it may return \code{NA}, second
  because it does not give the number of \emph{allowed} cores, and third
  because on Sparc Solaris and some Windows boxes it is not reasonable
  to try to use all the logical CPUs at once.
}
\examples{
detectCores()
detectCores(logical = FALSE)
}
